1. Field of the Invention.
The present invention relates to a heating system, and a method of operation thereof, which utilizes a three conductor heating element, and, more particularly, to an electric blanket which incorporates a heating element.
2. Description of the Related Art.
Blankets are woven from a variety of materials including wool, cotton, synthetics, and various animal and vegetable fibers. Blankets are used as a shawls, bed coverings and horse coverings. The blanket making of primitive people is one of the finest remaining examples of early domestic artwork. For example, the blankets of Mysore, India, are famous for their fine, soft texture. The loom of the Native American, though simple in construction, can produce blankets so closely woven as to be waterproof The Navaho, Zuxc3x1i, Hopi, and other Southwestern Native Americans are noted for their distinctive, firmly woven blankets. The Navahos produced beautifully designed blankets characterized by geometrical designs woven with yarns colored with vegetable dyes. The ceremonial Chilcat blanket of the Tlingit of the Northwest, is generally woven with a warp of cedar bark and wool and a weft of goats"" hair. Blankets, like society, have changed significantly over the years and it was in the 20th century that the electric blanket, with electric wiring between layers of fabric, gained wide popularity.
The direct conversion of electric energy into heat was first described by the English physicist James P. Joule. According to Joule""s law, a conductor carrying a current generates heat at a rate proportional to the product of the resistance of the conductor and the square of the current. It is the use of this principle, of applying electrical energy to a distributed resistance incorporated in a blanket, which provides warmth to the user. Joule""s law also points to a potential problem, if resistance is locally increased, in a distributed resistive element, more heat is produced in that localized area causing a local hot spot.
In spite of the advantages to the users of electric blankets, consumers have voiced concerns in several areas including EMF effects, potential for electrocution and as potential fire hazards. Manufactures of electric blankets have addressed consumer concerns with scientific studies, incorporation of safety features and marketing techniques. Regardless of the manufacturer""s care in manufacturing electric blankets, localized hot spots can occur in an electric blanket as a result of either a manufacturing defect, handling damage or consumer misuse. A localized hot spot in an electric blanket may cause damage to the electric blanket, property loss and/or injury to the user.
A design utilized by some manufacturers involves helically winding conductors in the heating element; however, helical winding of conductors adds greatly to the cost of a heating element.
What is needed in the art is an electric blanket with a heating element that is economical to manufacture, is safe and provides disconnection if a localized heating problem occurs.
The present invention provides a three wire heating element, two wires electrically resistive for the production of heat and the third wire sensing the temperature thereof and a connection to one of the electrically resistive wires; an electrical insulator surrounding and separating the three wires which melts at a predetermined temperature; and a control system which is coupled to the third wire to disconnect power from the two electrically resistive wires if either comes into contact with the third wire.
The invention comprises, in one form thereof, a heating element, including a sensor conductor, two resistive conductors, with one resistive conductor arranged on one side of the sensor conductor and an other resistive conductor arranged on an other side of the sensor conductor such that the sensor conductor and the two resistive conductors are substantially parallel and plastic electrical insulation surrounding the sensor conductor and each resistive conductor.
The invention comprises, in another form thereof, a heating blanket, including a blanket, a heating element routed within the blanket, a heating element including a sensor conductor, two resistive conductors, with one resistive conductor arranged on one side of the sensor conductor and an other resistive conductor arranged on an other side of the sensor conductor such that the sensor conductor and the two resistive conductors are substantially parallel and plastic electrical insulation surrounding the sensor conductor and each resistive conductor and a control device electrically coupled with the sensor conductor and the resistive conductors.
An advantage of the present invention is that if a heating conductor overheats, electrical power thereto is removed.
Another advantage is that localized overheating of a resistive conductor is detected.
Yet another advantage is that the temperature at which a localized heating problem is detected is predetermined by the selection of the melting temperature of the electrical insulation.
A further advantage is that the heating element is easily manufactured as three parallel wires electrically separated by electrical insulation.